Molecular structure and hydrolytic stability of polyurethane acetals

1076

V. V. PCHELINTSEVet al.

12. N. K. BONETSKAYA, V. I. IRZHAK, A. M. YEL'YASHEVICH and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 222: 140, 1975 13. V. V. KORSHAK and Yu. V. KORSHAK, Vysokomol. soyed. A26: 16, 1984 (Translated in Polymer Sci. U.S.S.R. 26: 1, 16, 1984) 14. M. EDEN, Proceedings of the Fourth Berkeley Symposium on Mathematical Statistics and Probability, Ed. J. Neyzman, Vol. 6, p. 233, Berkeley, 1961 15. H. P. PETERS, D. STAUFFER, H. P. HOLLTERS and K. Z. LOEWENICH, Physik B. 34: 399, 1979 16. R. WILSON, Vvedeniye v teoriyu grafov (Introduction to Graph Theory)• 208 pp., Moscow, 1977 17. L. D. LANDAU and Ye. M. LIFSHITS, Mekhanika (Mechanics) 3rd edition, corrected and supplemented, p. 127, Moscow, 1973 18. K. SOLC and W. H. STOCKMAYER, J. Chem. Phys. 54: 2756, 1971 19. W.L. MATTICE, Macromolecules 13: 506, 1980 20. P. G. KHALATUR, Vysokomol. soyed. A21: 2687, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 12, 2968, 1979) 21. C. J. C. EDWARDS, D. RIGBY, R. F. T. STEPTO and J. A. SEMLYEN, Polymer 24: 395, 1983 22. K. FREYDENBERG and A. C. NEISH, Constitution and Biosynthesis of Lignin, 120 pp., N. Y., 1968 23. J. GRAVITIS and P. ERINS, J. Appl. Polymer Sci. Appl. Polymer Symp., 37, 421, 1983

PolymerScienceU.S.S.R.Vol. 29, No. 5, pp. 1076-1081,1987 Printed in Poland

0032-3950/87 $10.00+.00 (~ 1988 Pergamon Press pie

MOLECULAR STRUCTURE AND HYDROLYTIC STABILITY OF POLYURETHANE ACETALS* V• V. PCHELINTSEV, A. YU. SOKOLOV, YU. G. KAMENEV, V. A. DRACH, V. N. RYZHOVA a n d G. YE. ZAIKOV Lebedev All-Union Synthetic Rubber Research Institute Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Received 29 October 1985)

The dependence of the MD of non-isocyanate polyurethane acetals on the methods o f their synthesis, nature and ratio of the starting co-monomers, depth of hydrolysis and the type of stabilizer of hydrolytic degradation has been studied. The optimal conditions of synthesis have been determined. The nature of the active centres of polymerization is discussed. The hydrolytic degradation of polyurethane acetals proceeds by the law of chance. The possibility of inhibiting hydrolytic degradation of the polyurethane acetals with the aid of hydrophobizing additives is demonstrated. * Vysokomol. soyed. A29: No. 5, 970-974, 1987.

Molecular structure and hydrolytic stability of PUAs

1077

SPECIALISTS engaged in the processing and use of rubbers are now paying increasing attention to polyurethane acetals ( P U A s ) [1]. As is known, the properties o f polymers are determined by their M D . T o g e t h e r with the problem o f regulating the M D there is the problem of stabilizing and controlling the properties o f the polymers since during storage and processing o f P U A s degradative processes occur [2-4] and the introduction o f stabilizers into the polymer only inhibits but does not completely suppress degradation [5-8]. The present w o r k is undertaken to study the dependence of the M D on the m o d e o f synthesis o f P U A s , the nature and ratio o f the starting c o m o n o m e r s , depth o f hydrolysis and the type o f stabilizer of hydrolytic degradation. The test objects were the products of the reaction of polyattachment of the divinyl ester of diethylene glycol (DVEDEG) to the urethane glycols N,N'-l,4-xylidene-bis-hydroxyethylcarbamate (UG-2) or N,N'-l,2-ethylene-bis-hydroxyethylcarbamate (UG-6) and to the polyester diol, polyethylene glycol adipate (P-6) and/or the monoester diol bis-(p-hydroxyethyl) terephthalate (BOET) taken in different molar ratios: (1.0 UG-2 : 0.2 P-6): 1.2 DVEDEG (1); (0'2 UG-2 • 1.0 P-6) : 1.2 DVEDEG (2); (0.1 UG.2 : 0'2 P-6): 1.2 DVEDEG (3); (l.0 UG-6 : 0.2 P-6): 0-2 BOET) : 1-2 DVEDEG (4); and (1.0 UG-6 : 1'0 P-6 : 1.0 BOET) : 1.2 DVEDEG (5). The modes of synthesis of the PUAs (l-5) are described in [9]. The MD of the polymers was studied by the GPC method with a Waters-200 chromatograph with four Styrogel columns in succession with porosity 3 x l0 s, 3 × 10z; 50 and 20 nm. The eluent was THF, elution rate 1 ml/min and temperature 298 K. The mean MM and polydispersity coefficients were evaluated from the universal calibration relation between the retention volume V~, MM and the intrinsic viscosity [q]: log (M[r/])~= 15-59 -0.2838Va obtained with the use of narrowly disperse standards and the Mark-Kuhn-Houwink equation [r/]=4.85 x 10-4M°'695, ensuring the best match between the experimental and calculated M values. The coefficients in the Mark-Kuhn-Houwink equation were det ermined by the method of least squares [10]. From the calibration relation and also the chromatogra ms of the initial comonomers UG-2, UG-6 and P-6 and BOET we evaluated MM of the impurities detected in the samples 1-5 and their content. Hydrolysis of the PUAs in plates measuring 25 × 10 x 2 mm and in films 40 pm thick was in aqueous solutions of hydrochloric acid of different concentrations at 293 K by the technique in [2]. The technique of measuring [q] is given there. The gel c h r o m a t o g r a m s of the P U A s correspond to a wide p o l y m o d a l M D (Fig. 1) where together with the main substance (peaks with retention volume 42, 43, 41.5, 41 and 44 counts respectively) the high molecular and low molecular (peaks with retention volumes 53.5 and 54"5 counts corresponding to UG-2, B O E T and U G - 6 respectively) impurities and P-6 (46.7 counts) are contained. The polvdispersity coefficient o f the P U A s according to Table 1 ranges from 4 to 17 and a l t h o u g h exclusion o f the low molecular weight fractions reduces it to 3-5, this value is m o r e than double the polydispersity value characteristic o f p o l y a t t a c h m e n t products for high values o f the degree o f completion of the reaction. Consequently, in the conditions o f P U A s synthesis the p o l y a t t a c h m e n t reaction is not complete. Let us consider f r o m the examples in Table 1 some factors influencing the character o f the molecular structure o f the PUAs. Thus, the single-stage m e t h o d o f synthesizing

1078

V. V. PCHELINTSEV e t al.

a #

Y..AAz

b

4 1

l

35

#5

5$

#5

Vn

countsS5

-

7

FIG. 1. Gel chromatograms of PUAs 1-5 (a) and their initial comonomers P-6 (1), UG-2 (2), UG-6 (3) and BOET (4) (b).

the polyurethane acetals 1-5 unlike the two-stage (samples la and lb) yields polymers with higher MM, i.e. with higher degrees of polyattachment; the sequence of the stages in the two-stage synthesis of la and l b is perceptibly reflected in the M M values and the degrees of completion; the high content of P-6 in the composition of the PUAs promotes

TABLE 1. M D CHARACTERISTICS OF THE POLYMERS

[q], dl/g Pol3mer

Mw/Mn

Share of low molecular weight fractions, wt. %

3050 2296

5.3 6.9

1"4

7831 7574

2372 1582

3.3 4.8

3"4

0.14 0"11

3940 3092

1239 465

3.2 6"7

22"9

0"18

0"31 0"30

12386 11906

5184 3192

2"4 3"7

4'0

0"34

0"55 0"52

28184 26629

8667 3344

3"7 8'0

5"6

0.58

0.50 0-47

25891 24297

5163 1467

5'0 16"6

6.0

0"14

0"23 0.21

8009 7538

2790 1857

2"9 4"1

6.2

experiment

calculation

0"38

0"35 0-35

16091 15854

la

0"15

0-22 0"22

lb

0"09

Note. The numerator gives the values obtained with reference to the share of low molecular weight fractions and the denominator without reference to them. Samples la and lb were obtained by two-stage methods. Sample la by the scheme P - 6 + D V B D E G ~ przpolymer; prepolymer + (UG-2 + DVEDEO) -~ la; ~arnple lb by the schcm© UO-2 + D V E D E O --, prePolymer; prepolymer + 0P-6 + DVEDEO) ~ lb.

Molecular structure and hydrolytic stability of PUAs

1079

the formation of polymers with narrower MD (samples 2, 3 and 5). The explanation for these phenomena must evidently be sought in the mechanism of the polymerization process. Earlier [1 l] with change in the content of the polyester in the PUAs from 0.1 to 0.5 moles the extremal dependence of It/] on composition was found with a maximum for the comonomer ratio (UG-2 : P-6) : DVEDEG =(1.0 : 0.2) : 1.2. The extremal change TABLE 2. KINETICS OF HYDROLYSIS OF

HCI, moles/l.

min -

0"4

0'7

5 10 21 25 5 7 10

PUA-1 FILMS IN AQUEOUS HCI SOLUTION AT 2 9 3 K

[~/], dl/g experiment[ calculation O.38 0"35 0'34 0"33 0.26 0"29 0"14 0"20 0"11 0'13 0'33 0.30 0.24 0'29 0.13 0"18

Mw/M. 16,091 14,364 12,226 6990 3518 13,020 12,328 5602

3050 2826 2618 2189 1356 2604 2512 1930

5"3 5"2 4"7 3"2 2"6 5'0 4'9 2-9

k x 107, sec- t

0"90+_0'06

1.74+_0.12

in [r/] was related to two superposed effects: increase in the M M of the polymer as a result of the large length of the material P-6 chain for an identical molar ratio of reactants; slowing of the polymerization rate through fall in the total polarity of the reaction mass from 5=20.0 to e=10.5 with increase in the P-6 content from 0.1 to 0"5 moles, which is characteristic of all polycondensation proceses [12]. The last factor, in our view, plays a decisive role in the formation of the molecular structure of the PUAs. It is confirmed by the results of the dependence of M M and the degree of completion on the sequence of the stages in the two-stage synthesis for the polymers la and lb. In this case e of the system determines the nature of the active polymerization centres. While at the first stage of formation of la probably the ion-pair mechanism of polyattachment predominates (for the system ~ < 10) at the first stage of obtaining lb the free ion mechanism is realized (5> 15). It may be assumed that for the ion-pair mechanism of polymerization (for example, in the cases of samples 2, 3 and 5) homopolymerization and cyclization of DVEDEG are suppressed. Hydrolysis of PUAs in acid media was accompanied by change in their MD (Table 2) with fall in the mean MM. The MD tended to the most probable value and the value of the polydispersity coefficient approached two. Such a change in the MD is characteristic of degradation occurring by the law of chance [13] which obeys the function described by the equation

ln(\ - m.d

m.o/

where -'~',o and ~ , , are the number average M M values of the initial and degraded polymer respectively. The values of the constants of hydrolytic degradation k calculated from this equation are presented in Table 2.

1080

V . V . PCHELINTSEVet a l

The random mechanism of degradation of the polyurethane acetals is also indicated by the linear dependence of In k on the acidity function Ho (Fig. 2) with the tangent of the angle of slope equal to unity. Earlier it was reported [4] that the hydrolytic degradation of PUAs essentially results from acid hydrolysis of the acetal bonds on the junctions of UG-2 and P-6 and is ac-

log ~ -70

-6'5

// S I

0

I

35

4E

Ho O.E

5E VR , counts

F:o. 2 FiG. 3 FIG. 2. Dependence of log k on the acidity function (sample 1, films, 293 K). Fro. 3. Gel chromatograms of the initial sample of PUA-1 (1) and samples obtained as a result of degradation for 60 (2) and 120 min (3).

companied by ruptures of the material chain and release of acetaldehyde. From the chromatograms of the hydrolysed samples I (Fig. 3) it follows that the acetal bonds lying next to the urethane fragments'are most readily hydrolysed (with increase in the depth of hydrolysis UG-2 accumulates in the volume of the sample). It is known [14] that urethane fragments form the basis of the amorphous phase of sample 1 readily accessible to water. The acetal bonds next to P-6 are less accessible since they are hidden in the crystalline phase of polyethylene adipate. This fact is indirectly confirmed by the difference detected (with increase in the depth of hydrolysis) between the calculated and measured values of [7] (Table 2), characteristic of PUAs with high P-6 content (Table 1, samples 2, 3 and 5). The values of the k constants obtained for the samples investigated differing in the degree of crystallinity K are given below Sample K, % kx 108, sec-1

1 2 3 4 17.5 34 38 < 10 3.16+0.01 0.25+0.01 0.071+0.0031 10.0+6.0

5 25 0.29+0.002

As is clear the hydrolytic stability of the PUAs depends on their nature and composition and declines in the series 3 > 2 1 > 5 > 1 > 4 , i.e. in the direction of decrease in the fraction of the crystalline phase in these polymers. Since the hydrolysis of acetures in real conditions (storage in briquettes at 293 K) proceeds in the internal diffusion-kinetic region [4] and depends on structural-kinetic factors it may apparently be retarded with the aid of hydrophobizing additives well

Molecular structure and hydrolytic stability of PUAs

1081

c o m p a t i b l e with the p o l y m e r . Such additives m a y be the m e t a l salts o f stearic acid, acetyl a c e t o n a t e a n d c a r b a m a t e [5-8]. The effect o f a d d i n g 5 wt. % o f the salt on the r a t e o f h y d r o l y t i c d e g r a d a t i o n ( c h a r a c terized b y the values o f t h e i n d u c t i o n p e r i o d tind a n d t h e c o n s t a n t k) o f plates o f the sample 1 in 1 M HCI s o l u t i o n at 293 K is i l l u s t r a t e d by the following p a r t i c u l a r s : Salt tjnd, min k x 108, sec --~

Acetyl acetonate Cu 28 2'32__+0"04

Diethyldithiocarbamate Zn 38 3'18 .+0'08

Stearate Ni 46 2-21 +0"11

F o r the s t a r t i n g m a t e r i a l ( w i t h o u t additives) t ~ . a = 0 a n d k = ( 3 . 1 6 ) + 0 . 0 1 x 10 - s sec- i. These c o m p o u n d s a c t u a l l y exert an inhibiting effect on h y d r o l y t i c d e g r a d a t i o n a n d their effectiveness ( a c c o r d i n g to the length o f the i n d u c t i o n p e r i o d ) decreases in t h e series Ni s t e a r a t e > Z n c a r b a m a t e > Cu acetyl a c e t o n a t e . Translated by A. Ce.oz~" REFERENCES

1. L. Ya. RAPPOPORT, G. N. PETROV, I. I. TROSTYANSKAYA and O. P. GAVRILOVA, Kauchuk i rezina, 1, 25, 1981 2. I. D. SOKOLOVA, V. V. PCHELINTSEV, Yu. A. L'VOV, I. I. TROSTYANSKAYA, L. Ya. RAPPOPORT and G. N. PETROV, Prom-st. SK, 11, 7, Moscow, 1977 3. V. V. PCHELINTSEV, I. D. SOKOLOVA, V. P. VOLKOV and V. D. SEMENOV, 1bid. 6, 16, Moscow, 1979 4. V. V. PCHELINTSEV, A. Yu. SOKOLOV and A. L. KLEBANSKII, Sintez novykh modifikatorov dlya elastomerov (Synthesis of New Modifiers for Elastomers), p. 108, Moscow, 1983 5. V. V. PCHELINTSEV, I. D. SOKOLOVA, N. A. SILINA and V. D. SEMENOV, U.S.S.R. Pat. 729219; B. I., 15, 100, 1980 6. V. V. PCHELINTSEV, I. D. SOKOLOVA and N. A. SIL1NA, U.S.S.R. Pat. 729220; B. I., 15, 100, 1980 7. V.V. PCHELINTSEV, I. D. SOKOLOVA, N. A. SILINA, V. D. SEMENOV and Yu. A. L'VOV, U.S.S.R. Pat. 787439; B. I., 46, 103, 1980 8. V. V. PCHELINTSEV, I. D. SOKOLOVA, V. D. SEMENOV, A. Yu. SOKOLOV and A. L. KLEBANSKII, U.S.S.R. Pat. 870420; B. I., 37, 138, 1981 9. L. M. VOLKOVA, G. V. KOROLEV, F. I. DUBOVITSKII, I. I. TROSTYANSKAYA, L. Ya. RAPPOPORT, G. N. PETROV, M. F. SHOSTAKOVSKII, R. D. YAKUBOV and S. M. MAKSIMOV, U.S.S.R. Pat. 518506; B. I., 23, 78, 1976 10. B. G. BELEN'KII and L. Z. VILENCHIK, Khromatografiya polimerov (Chromatography of Polymers), p. 202, Moscow, 1978 11. I . I . TROSTYANSKAYA, Dissert. Cand. Chem. Sci., p. 77, VNIISK, Leningrad, 1977 12 S. G. ENTELIS and R. P. TIGER, Kinetika reaktsii v zhidkoi raze (Kinetics of Reactions in the Liquid Phase), p. 394, Moscow, 1973 13. AI. AI. BERLIN and N. S. YENIKOLOPYAN, Vysokomol. soyed. A10: 1475, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 7, 1706, 1968) 14 G. Ye. NOVIKOVA, A. B. KORENNAYA, I. V. BARANETS and I. I. TROSTYANSKAYA, Kauchuk i rezina, 2, 19, 1981